Looks like this is one of the ones moved to the modules-extra package (this split is kernel specific, and wasn't done on the PC kernel which probably explains why this adapter works on your PC installation of Ubuntu Server). If you do the following:
$ sudo apt install linux-modules-extra-raspi
You should then be able to modprobe usbmon. I'll move the bug over to linux-raspi as it's technically a kernel issue.
Looks like this is one of the ones moved to the modules-extra package (this split is kernel specific, and wasn't done on the PC kernel which probably explains why this adapter works on your PC installation of Ubuntu Server). If you do the following:
$ sudo apt install linux-modules- extra-raspi
You should then be able to modprobe usbmon. I'll move the bug over to linux-raspi as it's technically a kernel issue.